Computer terminal and digital television display systems commonly employ a technique known as "bitmapped" graphics. A "bitmap" is a storage array of bits ("1s" and "0s") corresponding to the intensity and/or color pattern to be displayed at the various locations on a display screen. A display system uses these bits to properly modulate the electron beam of the cathode ray tube, thereby displaying a desired image. Most commonly, these bits are stored in memory as an array, ordered in a left to right sequence within a top to bottom sequence (sometimes referred to as a raster scan order). As the display screen is scanned by the electron beam one horizontal line at a time from top to bottom (and from left to right within each horizontal scan line), the bitmap data can be readily accessed sequentially. Changes in the displayed image are accomplished by changing the data in the bitmap. The display system then displays changed data on subsequent scans of the display device.
Display systems in which a plurality of independent sets of information are displayed concurrently on one display device, each set in its own individual "window," are well known. In the case of computer-based video display systems, each set of information (i.e., the contents of each window) is often associated with an individual program executing in a multi-tasking computer system. Thus, a computer user can simultaneously execute multiple applications and may view the activity of, or otherwise interact with, each of these programs concurrently. In television systems, one or more of the windows may contain video image displays. For example, several such television pictures may be "tiled" on a single television screen, allowing the viewer to watch two or more programs concurrently.
In both the computer display and television examples each window may be independently located within the overall display area. Information to be displayed in each of the windows typically consists of a set of information elements (pixels) arranged in a field of arbitrary, rectangular dimensions. Furthermore, the information in each such window may be either periodically or continually updated, whereupon the display system re-displays the updated information as necessary. This re-display is achieved by the repetitive scanning of the display device, typically a cathode ray tube (CRT). On each scan, the current pixel data of each window is displayed at the appropriate location in the display area.
The concept of visually "overlapping" windows is also well known. Where windows overlap, one window may appear to be in front of another window. As such, the windows in front partially or wholly obscure the visibility of the windows in back. For example, in television systems one or more of the video images may be imbedded in others (a technique commonly known as "picture-in-picture"). To achieve the appearance of overlapping windows, all windows are ordered in a display priority. Wherever more than one window is to occupy the same location in the display area, the pixels of a higher priority window are displayed instead of the pixels of lower priority windows. Moreover, the priority ordering of the windows may be modified over time. Such a modification effects the resultant display by altering the appearance of which windows are in front of which other windows (i.e., by changing which portions of windows are visible and which portions are obscured). Typically, there is also a background "window," which occupies the entire display area and is permanently assigned the lowest display priority.
One approach to the display of overlapping windows employs a selection algorithm to determine which data to display at each location on the display screen in order to properly produce the desired image. Such an algorithm can be readily implemented in hardware, resulting in extremely efficient display systems. The display hardware may be provided with pointers to the individual window bitmaps, as well as information regarding the location and display priority of each window. The hardware then determines, for each location on the display screen, from which bitmap the pixel to be displayed should be retrieved, and which pixel data within that bitmap should be used.
The use of rectangular windows simplifies the handling of bitmapped data and the manipulation of the windows, resulting in efficient display systems. In particular, the position and size information for a rectangular window is commonly represented by only four numbers. These may be a horizontal location ("X") coordinate and a vertical location ("Y") coordinate to indicate the position of the window in the display area, and a width and height to indicate the size of the window. Equivalently, the four numbers used to identify the position and size information may be a horizontal start location ("Xstart"), a horizontal end location ("Xend"), a vertical start location ("Ystart"), and a vertical end location ("Yend"), thus defining the four corners of the window.
Window display systems typically display the data from individual window bitmaps by determining, for each pixel location in the display area, the window of highest priority which includes that pixel location. This determination is straightforward when the windows are rectangular. For example, when the four corners are used as described above to locate each window, that window includes a given X,Y pixel location if and only if X is not less than "Xstart" nor more than "Xend," and Y is not less than "Ystart" nor more than "Yend." Furthermore, once the appropriate choice of windows is made, retrieving the correct pixel data for the given pixel location in the display area is equally straightforward. Since the data is stored as a conventional rectangular array, a simple mathematical calculation is sufficient to locate the appropriate pixel data.
It would often be advantageous to include the ability to display an image contained in a rectangular window without displaying the surrounding portions of that window. In effect, these surrounding portions of a foreground window would be made "transparent," so that the images "behind" these portions of the foreground window become visible. From the viewer's perspective, such a capability would provide the functional equivalent of displaying "windows" of arbitrary shape. In multimedia applications, for example, it is desirable to construct video frames composed of a background scene plus a number of independently controllable objects. One such illustration would be a background consisting of an undersea scene, with many independently movable fish overlaid on that scene. If each fish were displayed as a complete rectangular window, the area of each of these windows which is outside the border of the image of the fish itself would inappropriately obscure part of the background (or a view of part of another fish) "behind" it. By displaying only the fish image, and not the surrounding portions of the window which contains it, a much more realistic scene would be displayed.